Help needed with the Western Roman Empire (RTW:BI)

[The Dark]

I never use artillery due to the huge movement penalties it inflicts and because it is very ahistorical.

Ahistorical is somewhat debatable. Trajan's Column shows artillery moving with the legion, and according to De Re Militari, the legion had 10 onagers and about 60 ballistae. Assuming it's correct about the legion having artillery, the onagers were probably not assembled until needed for a siege, due to weight, but a ballista was about the right weight to be pulled in a cart (as the column shows), although recoil would make the carroballista as it exists in the game unlikely. It would probably have been drawn on a cart to the battlefield, then unloaded for use.

[Thanas]

I never use artillery due to the huge movement penalties it inflicts and because it is very ahistorical.

Ahistorical is somewhat debatable. Trajan's Column shows artillery moving with the legion, and according to De Re Militari, the legion had 10 onagers and about 60 ballistae. Assuming it's correct about the legion having artillery, the onagers were probably not assembled until needed for a siege, due to weight, but a ballista was about the right weight to be pulled in a cart (as the column shows), although recoil would make the carroballista as it exists in the game unlikely. It would probably have been drawn on a cart to the battlefield, then unloaded for use.

You are talking about a classical Roman legion, which were disbanded due to the reforms of Diocletian and Constantine. The late Roman legion, composed of about 1200 men, did not have an artillery detachement, there were specialized legions for this.